Welcome! I've got two Cavaliers, a 98 auto and an 02 manual, both 2200's. Someone here has a heavily modified ecotec that gets mid 50's on the highway, but it's stripped. What engine and trans do you have? If you've made it to 175Kmi without spinning a rod bearing, you probably have the 2200
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My 98 could do 42mpg highway on pure gasoline. Since I got my 02, I can only get E10, and could get about 38mpg highway with that in stock form. With a lean mixture, I'm back in the low 40's. The only aero mods I've done to that is remove the mud shields on the front fenders. I haven't measured the results, but they can't be doing me any favors.
As with any car, I'd recommend switching to synthetic fluids, then catching up with maintenance if you're not already there. Then do all the easy things that help increase mpg on any car (inflate tires, etc). Then we get to the more interesting parts.
